Hitherto, a method for forming a multilayer coating film by a 3-coat 2-bake (3C2B) process is widely used as a method for forming a coating film on automobile bodies. This method sequentially comprises the following steps after applying an electrodeposition coating composition to a substrate, followed by heat-curing: application of an intermediate coating composition→curing by baking→application of a base coating composition→preheating (preliminary heating)→application of a clear coating composition→curing by baking. In general, this 3-coat 2-bake process is widely used for forming a coating film with a so-called metallic color by using a base coating composition containing an effect pigment (a luster pigment).
However, in recent years, for the purpose of saving energy, consideration has been given to omitting the bake-curing step that is performed after applying the intermediate coating composition, and using a 3-coat 1-bake (3C1B) process sequentially comprising the following steps: application of an intermediate coating composition→preheating (preliminary heating)→application of a base coating composition→preheating (preliminary heating)→application of a clear coating composition→curing by baking. From the viewpoint of reducing the environmental pollution caused by the vaporization of organic solvents, the establishment of a 3-coat 1-bake process using aqueous coating compositions as the intermediate coating composition and the base coating composition is particularly desired. It is also desired, for further energy savings, to omit the preheating step that is performed after applying the intermediate coating composition.
However, in the above-described 3-coat 1-bake process using an aqueous intermediate coating composition and an aqueous base coating composition, a resulting coating film is likely to have insufficient water resistance due to the use of a water-soluble or water-dispersible resin, as well as insufficient smoothness and distinctness of image due to the formation of a mixed layer from the aqueous intermediate coating composition and the aqueous base coating composition. In particular, when the preheating step that is performed after applying the intermediate coating composition is omitted, significant mixing of the aqueous intermediate coating composition and aqueous base coating composition occurs at the interface between the aqueous intermediate and aqueous base coating layers, resulting in the formation of a coating film with considerably deteriorated smoothness and distinctness of image.
In order to solve such problems, Patent Literature (PTL) 1 discloses a 3-coat 1-bake multilayer coating film forming method comprising sequentially applying an aqueous first colored coating composition, an aqueous second colored coating composition, and a clear coating composition. The method of PTL 1 comprises: applying, as the aqueous first colored coating composition (A), an aqueous colored coating composition (A) that contains 50 to 90 parts by weight of an urethane emulsion (a), and 10 to 200 parts by weight of a pigment component (c), relative to 100 parts by weight of the solids content of a resin component, and that has a solids content of 45 to 65 wt %; subjecting the resulting coating film to setting at an ordinary temperature, without air blowing or preheating; adjusting the viscosity of the coating film to 1×103 Pa·s or higher (at 20° C., and a shear rate of 0.1 sec−1); and applying the aqueous second colored coating composition having a solids content of 15 to 50 wt %. In this method, PTL 1 further discloses that, no matter which portion (e.g., a horizontal or vertical portion) of a substrate the compositions are applied, a multilayer coating film can be obtained that has excellent finished appearance, and that does not have sagging, a mixed layer formed from the compositions, etc. Even in this multilayer coating film-forming method, however, there was a case where the resulting coating film was insufficient in smoothness and distinctness of image.
PTL 2 relates to a method for forming a multilayer coating film, comprising the steps of: applying an aqueous intermediate coating composition to a substrate to form an uncured intermediate coating film; applying an aqueous base coating composition to form an uncured base coating film; applying a clear coating composition to form an uncured clear coating film; and heating the uncured intermediate coating film, the uncured base coating film, and the uncured clear coating film to simultaneously cure these three coating films. However, PTL 2 discloses that it is preferable to perform preheating after the application of the aqueous intermediate coating composition, considering the finished appearance of the resulting film. This requires the heating step to be performed three times, and the method is therefore insufficient in terms of reduction in steps.